There are important economic and environmental benefits associated with an increase in the efficiency of electric power plants. It is particularly desirable to have such efficient power plants capable of burning a wide range of fuels with minimal pollution. Another highly desirable feature is the ability to function economically over a wide range of sizes because some of the renewable (biomass) fuels are most suited to small power plants while other fuels, such as coal and natural gas, are better suited to large power stations. While many of these features have been achieved by existing power plants, the quest for efficiency has resulted in highly complex power plants which are economical only in large sizes and do not have the flexibility to burn multiple fuels.
Pressurized fluidized bed combustion is a known method for clean fuel burning, particularly for burning solid fuels. It provides the ability to capture fuel contaminants such as sulfur in most coals through chemical reaction with sorbents added during combustion. The pressurized fluidized bed further maintains combustion temperatures within the range at which the contaminants can be effectively absorbed and the formation of toxic nitrogen oxides can be limited. Fluidized bed combustion is thus able to provide environmentally acceptable emissions for the combustion of most fuels.
A pressurized fluidized bed combustor can be of either the circulating or bubbling type. A number of differences exist between the two types. First, the circulating bed operates at higher fluidizing velocities than the bubbling bed, so that the combustor can be more compact, leading to easier feeding and better distribution of fuel, and of sorbent if needed. Because the circulating combustor is smaller, modular construction and factory assembly become feasible, which lowers capital cost. Second, the circulating combustor enables simpler load following through controlling the recirculation rate of bed material without the need to transfer bed material to and from additional bed material storage vessels as required by the bubbling bed. Finally, the circulating combustor uses an external heat exchanger, located outside the harsh environment of the combustion bed, whereas the bubbling bed uses in-bed heat exchanging equipment.
The circulating pressurized fluidized bed combustor has been used for power generation in combined cycles coupling a simple open Brayton-cycle gas turbine with a Rankine-cycle steam turbine. Although the Brayton cycle is relatively efficient in power conversion, the overall efficiency of such plants is limited by the low thermal efficiency of the Rankine cycle because of the large latent heat loss during the steam condensation phase of the steam turbine power cycle.
To improve combined cycle efficiency, advanced designs use very high excess air, higher combustion pressure, and higher gas turbine inlet temperature to increase the thermal load ratio of the gas turbine to the steam turbine and to increase the efficiency of the gas turbine cycle. To obtain higher turbine temperature, a second combustor of conventional type is provided to further heat the flue gas exhausted from the fluidized bed before it enters the gas turbine. The overall plant efficiency of such advanced combined cycles is improved, but remains limited by the low efficiency of the Rankine-cycle steam turbine. Another longstanding problem in combined cycle power plants is the presence of economies of scale. Because of the complexity of steam turbines and their associated equipment, steam turbines are economical only in relatively large size. This problem has seriously limited the commercial value of combined cycle power plants for such applications as independent power production or biomass plants that require small power output units.
A power conversion cycle without a steam turbine can be a solution to the problems facing the prior art fluidized bed combustor power plants. One approach is disclosed in U.S. Pat. No. 3,791,137, in which a power conversion cycle based on an open-cycle gas turbine and a closed-cycle helium turbine is used with a bubbling pressurized fluidized bed combustor. However, several significant technical and economic difficulties arise with this design, impeding its commercial feasibility. First, the heat exchanger which serves as the indirect heater for the closed-cycle helium turbine must be positioned within the combustion bed of the bubbling bed. Consequently, the heater outer surface (the combustion side) is attacked by the corrosive and erosive environment in the combustion bed. Furthermore, the helium flow of the closed Brayton-cycle gas turbine must be heated in the heater to a high temperature close to that of the bed combustion in order to obtain high thermal efficiency for the closed-cycle helium turbine. This results in very high temperatures for the heater material, accelerating the corrosive and erosive attack on the heater material. In such conditions, it is difficult for conventional, economic materials to provide an adequate lifetime for the heater. Second, the combustor must operate at a relatively low pressure, e.g., about six times atmospheric pressure. Higher pressures would result in reduced system thermal efficiency because of the lack of an effective means for controlling stack loss at higher pressures. The design given in U.S. Pat. No. 3,791,137 is thus unable to take advantage of the fact that higher pressures for combustion have the effect of reducing the size and thus the cost of the combustor and of lowering the pressure load and stress imposed on the construction material of the heater. Third, the overall cycle efficiency is limited by the requirement of the low temperature combustion in the fluidized bed for effective emission control. Finally, there are difficulties in mechanical and aerodynamic designs for helium turbines for small power plants with an electric output of about 20 megawatts. Designers have been unable to design relatively inexpensive small helium turbine prototypes, adversely affecting the use of closed-cycle helium turbines.